Capstan drives using steel wire rope or cables are well known devices. They provide excellent traction in many situations. Various capstan-type cable drive mechanisms have been devised, particularly for hoisting devices.
In some systems the cables pass around or between two or more rollers with tractive friction being provided by nip pressure between the rollers. Representative examples include devices disclosed in U.S. Pat. Nos. 3,227,420; 4,058,294; 4,113,237 and 4,294,429. Other hoist arrangements using multiple rollers are disclosed in U.S. Pat. Nos. 3,717,325 and 3,785,511.
Devices are also known in which a pinch roller mounted on a pivotable arm is used to hold the cable against the winding drum and/or to control the winding of the cable on the drum. Examples of such devices are disclosed in U.S. Pat. Nos. 2,625,373; 3,836,123 and 3,841,606.
However, conventional capstan drives are subject to a number of disadvantages. Typically, a very large drum is required, particularly where large loads must be moved or where loads must be moved through longer distances. Heavy loads require use of comparatively large diameter cable or wire rope, which in turn requires a large diameter capstan to avoid sharp bends which result in excessive bending stresses on the cable. Heavy loads also require a larger number of wraps around the capstan to provide the required friction to prevent slippage. On long pulls the cable "walks" sideways along the drum, thus requiring drums of substantial width.
If there are only a few wraps of cable, the cable may be permitted to side slip and the width of the drum may be reduced, but the pulling power is also reduced. Such side slipping arrangements increase the wear on the cable and also cannot be used where the need for a large pull strength requires a large number of wraps.
A problem is also encountered in starting up cable drive systems. Unless the cable is urged against the drum by tension or otherwise to provide adequate energizing friction between the cable and the drum, the drum may spin within the cable without drawing the cable in the desired direction.
Hydraulic cable pullers which operate by means of reciprocating grippers are also known. These devices are compact and very powerful, but because of their reciprocating motion, they are very slow and their operation is not especially smooth.
There remains a need for a cable drive mechanism which is not subject to the foregoing disadvantages.